// вначале семафор закрыт
newCustomer = new AutoResetEvent(false); 

// customer reception thread[s]
lock (queue) {
  if (queue.Size == maxSize)
    throw "нет мест";
  queue.Enqueue(сustomer);
  newCustomer.Set(); // open semaphore
}

// each barber's thread
while (true) {
  newCustomer.WaitOne(); // let one barber through and close
  lock (queue) {
    if (queue.IsEmpty) 
      continue;
    customer = queue.Dequeue();
  }
  Shave(customer);
}